1. Field of the Invention
The present invention relates to exposure apparatus of the type that employs a short-arc mercury-vapor lamp as an exposure light source for formation of the fluorescent screen of a color cathode ray tube.
2. Description of the Prior Art
As disclosed in Japanese Patent Examined Publication No. 47-31264, a spot light source device in which a secondary light source is obtained from a linear or rod-shaped light source has in the past been used as an exposure apparatus for formation of the fluorescent screen of a color cathode ray tube, and then a very high pressure mercury-vapor lamp is used as a primary light source.
Japanese Patent Examined Publication No. 43-28553 proposes an exposure apparatus employing a very high pressure mercury-vapor lamp of a so-called short-arc (electrode-load) type in which a cathode and an anode are oppositely disposed in an oval bulb charged with mercury vapor.
Such a short-arc mercury-vapor lamp which is used in the exposure apparatus has the following problem. Although the required exposure accuracy (the allowable offset amount of the arc center of the mercury-vapor lamp with respect to the center axis of the panel) is typically approximately 0.1 mm, the consumption rate of the emission end of the cathode of the mercury-vapor lamp varies by 0.1 mm/24 hr for an initial lighting period of up to about 100 hours and, subsequently, by 0.02 mm/24 hr after about 100 hours. As a result, the arc length of the mercury-vapor lamp increases in one direction and an error may occur in the position of each phosphor dot, and this adversely affects the configuration of the phosphor dots on the fluorescent screen of the panel.
In order to solve these problems, it has thus far been necessary that while an operator is looking through a viewfinder incorporated in a lamp house which accommodates the short-arc mercury-vapor lamp, he manually operates a micrometer head to perform fine alignment of the mercury-vapor lamp and correct the exposure accuracy of the arc center. However, the above-described setting has required a very long period of time.